Hydration-induced conformational changes in protonated 2,4-pentanedione in the gas phase

Abstract

Starting with H+[CH3C(O)CH2C(O)CH3] (denoted H+PD), the protonated diketone-water clusters H+PD(H2O) n (n = 1–3) have been characterized by density functional theory calculations in combination with vibrational predissociation spectroscopy to explore the conformational changes of a protonated bifunctional ion solvated by water in the gas phase. Theoretical calculations for H+PD revealed that the ion contains an intramolecular hydrogen bond (IHB), with two oxygen atoms bridged by the extra proton in an O—H+ … O form. Attachment of one water molecule to it readily ruptures this IHB, replacing the H+ by the H3O+ moiety. Further replacement of the IHB by two water molecules occurs at n = 2 and the −C(O)CH2C(O)- chain is fully opened (or unfolded) after transfer of the extra proton to the water trimer at n = 3. To verify the computational findings, infrared spectroscopic measurements were performed using a vibrational predissociation ion trap spectrometer to identify cluster isomers from the signatures of hydrogen bonded and non-hydrogen bonded OH stretching spectra of H+PD(H2O)2,3 produced in a corona discharge supersonic expansion. Besides open form isomers, evidence for the formation of water-bridged structures has been found for H+PD(H2O)3 at an estimated temperature of 200 K. A detailed illustration of the unfolding steps as well as the energy profiles for the evolution of a two-water bridge isomer from the protonated H+PD monomer are analysed pictorially (including both stable intermediates and transition states) in the present investigation.